1. Field of the Invention
The present invention relates to a method of modulating a feedback signal in an optical amplifier and, more particularly, to a method of modulating a feedback signal in an optical amplifier which employs a laser diode which can directly amplify light.
Optical amplifiers in practical use directly amplify light by applying excitation light to an erbium-doped fiber. It is necessary for these optical amplifiers to provide a method of correctly modulating a feedback signal (a supervisory signal) even if the characteristics of a laser diode deteriorate due to aging or changes in temperature.
2. Description of Prior Art
FIG. 3 is a block diagram showing a conventional optical amplifier, employing a laser diode, which can directly amplify light. This optical amplifier may serve as a repeater of a submarine cable. In FIG. 3, reference numeral 1 designates an optical multiplexer, 2 is an optical branching filter, 3 is a photodiode (PD), 4 is an automatic level controller (ALC), 5 is a band-pass filter (BPF), and 6 is a supervisory circuit.
Reference numeral 71 denotes a first drive circuit, 72 is a second drive circuit, 81 is a first laser diode, 82 is a second laser diode, 9 is a polarization coupler, and 10 is an erbium-doped fiber (EDF). The first drive circuit 71 and the first laser diode 81 are normally used, and the second drive circuit 72 and second laser diode 82 are spare elements. The first drive circuit 71 is coupled to a first laser diode 81 which is, in turn, coupled to a polarization coupler 9. Reference numeral 82 designates a second laser diode.
In the optical amplifier of FIG. 3, the erbium-doped fiber 10 transmits light. Part of the light is filtered out by the optical branching filter 2 and is converted into an electrical signal by the photodiode 3. The automatic level controller 4 controls the output light of the laser diode 81 or 82 through the drive circuit 71 or 72, such that the photodiode 3 provides a constant output corresponding to the output power of the optical amplifier. The output light of the laser diode 81 or 82 is supplied to the erbium-doped fiber 10 through the polarization coupler 9 and optical multiplexer 1, as shown by arrows.
FIGS. 5(a) and 5(b) are graphs explaining the compensation for the deterioration of the characteristics of the laser diode of the amplifier. FIG. 5(a) shows the relationship between the deterioration of the characteristics of the laser diode due to temperature fluctuations and aging and a bias current flowing to the laser diode controlled by the automatic level controller 4, and FIG. 5(b) shows the relationship between the deterioration of the characteristics of the laser diode due to temperature fluctuations and aging and the output power of the optical amplifier excited by the laser diode driven by the bias current controlled by the automatic level controller 4 of FIG. 3.
When the characteristics of the optical amplifier deteriorate due to aging or changes in temperature, the automatic level controller 4 monitors the strength of light in the erbium-doped fiber 10 through the optical branching filter 2 and photodiode 3 and controls the bias current flowing to the laser diode 81 (82), as shown in FIG. 5(a). Namely, when the characteristics of the optical amplifier deteriorate due to increased temperature or aging, the bias current applied to the laser diode 81 (82) is increased to maintain the output power of the optical amplifier at a constant level as shown in FIG. 5(b).
Referring back to FIG. 3, the output of the photodiode 3 is also supplied to the band-pass filter 5, which filters a predetermined band of the output, to provide a supervisory signal (SV). The supervisory signal is supplied to the supervisory circuit 6 to carry out predetermined control. The supervisory circuit 6 monitors, e.g., the conditions of a repeater of a submarine cable. The supervisory signal representing the result of monitoring is transmitted to a station by modulating the amplitude of a main signal by a drive circuit (a superimpose circuit) according to the supervisory signal SV.